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OpenAPI Down Convert

openapi-down-convert is a tool to down-convert an API definition document from OpenAPI 3.1 to OpenAPI 3.0.

Warning: This is not a fully robust tool. It does the minimal work necessary for OAS 3.1 API documents in order to support tools such as openapi-generator which do not support OAS 3.1. It only supports the OAS 3.1 features that the Apiture APIs use.

Warning: Down converting yields a loss in fidelity. Some API information is lost.

Installation and Synopsis

The tool is implemented with Node.js.

⤓ NPM Installation

npm i @apiture/openapi-down-convert
import { Converter, ConverterOptions } from './converter';
const options : ConverterOptions = { verbose: false,
                                     deleteExampleWithId: true,
                                     allOfTransform: false };
  const converter = new Converter(oas31Document, options);
  try {
    const oas30Document = converter.convert();
    ...
    }
  } catch (ex) {
    // handle the exception
  }

Since the use case for the converter is in build pipelines and CLI use, this operation is synchronous and does not use async/await/Promises.

⤓ Command Line

Install:

npm i -g @apiture/openapi-down-convert

then to use:

openapi-down-convert --input openapi-3.1.yaml --output openapi-3.0.yaml

Command line options:

Usage: openapi-down-convert [options]

Options:
  -i, --input <input-file>               A OpenAPI 3.1 file name. Defaults to "openapi.yaml"
  -o, --output <output-file>             The output file, defaults to stdout if omitted
  -a, --allOf                            If set, convert complex $ref in JSON schemas to allOf
  --authorizationUrl <authorizationUrl>  The authorizationUrl for openIdConnect -> oauth2 transformation
  --tokenUrl <tokenUrl>                  The tokenUrl for openIdConnect -> oauth2 transformation
  -d, --delete-examples-with-id          If set, delete any JSON Schema examples that have an `id` property
  --oidc-to-oath2 <scopes>               Convert openIdConnect security to oath2.
  --convertJsonComments                  If used, convert `$comment` in JSON schemas
                                         to `x-comment`. If omitted, delete
                                         all `$comment` in JSON schemas.
                                         (Use `--verbose` to log deletion
                                         to stdout)
  -s, --scopes <scopes>                  If set, this JSON/YAML file describes the OpenID scopes.
                                         This is an alias for --oidc-to-oath2
  -v, --verbose                          Verbose output
  -V, --version                          output the version number
  -h, --help                             display help for command

The verbose mode logs all changes to standard error output stream.

The tool returns a 0 status code upon success or a non-zero status code if it finds constructs that cannot be down-converted, such as using contentMediaType: application/octet-stream with a format other than binary, or if a schema has contentEncoding: base64 and has an existing format that is not already base64.

The tool only supports local file-based documents, not URLs. Download such files to convert:

openapi-down-convert --input <(curl -s https://my.host/path/openapi-3.1.yaml) \
                     --output openapi-3.0.yaml

OpenAPI Specifications Transformations

Here is a list of the transformations the tool performs:

⤓ openapi declaration

Change openapi: 3.1.x to openapi: 3.0.3

⤓ openIdConnect security definitions

Replace openIdConnect security definition with an oauth2 security requirement They are close enough, as far as code generation (such as openapi-generator) is concerned - it just means an Authorization: header must have a valid token.

Note: This conversion is only performed if the --oidc-to-oauth2 option (or it's alias, --scopes) is supplied. Use the other options to specify the authorizationUrl and tokenUrl for the oauth2 security definition.

TODO: Fetch the openIdConnect connection info and extract the authorization and token URLs from it.

    accessToken:
      type: openIdConnect
      description: ...
      openIdConnectUrl: 'https://auth.apiture.com/openidConnectDiscovery'

becomes something like:

accessToken:
  type: oauth2
  description: "OpenIDConnect authorization code flow via https://auth.apiture.com/openidConnectDiscovery"
  flows:
    authorizationCode:
      authorizationUrl: <authorizationUrl option>
      tokenUrl: <tokenUrl option>
      scopes:
        scope1: Allow the application to access your personal profile data.
        scope2: Allow the application to send email on your behalf.
        scope3: >-
          TODO: describe the 'scope3' scope.
        scope4: >-
          TODO: describe the 'scope4' scope.

The tool scans all the security objects in all the operations to build a list of the used scopes. The descriptions for the scopes should be be supplied in the scopes.yamlfile as simple scopeName: scope description pairs:

scope1: Allow the application to access your personal profile data.
scope2: Allow the application to send email on your behalf.

⤓ $ref object rewrites

For all schemas which contain a $ref object with siblings (description, other schema elements), replace the $ref: uri with allOf . For example:

myProperty:
  description: Blah blah
  $ref: '#/components/schemas/MyArray'

becomes:

mySchema:
  description: Blah blah
  allOf:
    - $ref: '#/components/schemas/MyArray'

This also applies to the schema used in parameters or in requestBody objects and in responses.

Note This transformation is disabled by default because it breaks openapi-generator 5.4 in cases where the referenced schema is an array. It generates Typescript types for such cases as

  myProperty: Array | null;
model/Model.ts:26:23 - error TS2314: Generic type 'Array<T>' requires 1 type argument(s).

which should be

  incompleteAccounts: Array<string> | null;

To enable, use the allOfTransform: true option in the Converter constructor or the --allOf command line argument. When disabled, the $ref is instead simplified to a JSON reference.

Other (non-JSON Schema) OAS 3.1 $ref objects can have description and summary. $ref for non-schema objects in OAS 3.0 cannot have description and summary. openapi-down-convert simply removes description and summary to yield a valid JSON reference as required by OAS 3.0. (The resulting OpenAPI will use the description in the $ref target.)

⤓ info.license.identifier

Remove info.license.identifier.

⤓ webhooks

Remove the webhooks object, if present.

⤓ JSON Schema related changes

OAS 3.0 uses an earlier JSON Schema version (JSON Schema Specification Wright Draft 00). The tool converts examples in schemas to a single example.

As a special case, if the resulting example includes an id, it is deleted if the --delete-examples-with-id CLI option is set. This addresses Spectral issue 2081.

⤓ Convert const to enum

Convert JSON Schema that uses const to an enum with one value. For example

components:
  schema:
     version:
        description: The API version.
        type: string
        const: '1.0.0'

becomes

components:
  schema:
     version:
        description: The API version.
        type: string
        enum:
          - '1.0.0'

⤓ Convert type arrays to nullable

If a schema has a type array of exactly two values, and one of them is the string 'null', the type is converted to the non-null string item, and nullable: true added to the schema.

For example:

    myResponse:
      title: My Response
      description: Response from an API operation
      type: [ object, 'null' ]
      allOf:
        ...

becomes

    myResponse:
      title: My Response
      description: Response from an API operation
      type: object
      nullable: true
      allOf:
        ...

and

    myResponse:
      title: My Response
      description: Response from an API operation
      type: array
      items:
        type: [ 'string', 'null' ]
        ...

becomes

    myResponse:
      title: My Response
      description: Response from an API operation
      type: array
      items:
        type: string
        nullable: true
        ...

This transformation does not handle more complex type array scenarios such as

   type: [ number, string, boolean, 'null']

To support that, the schema would need to be recast using oneOf, but this is not trivial due to other schema attributes that may be possible (properties, allOf etc.)

(Contributions welcome.)

⤓ Remove unevaluatedProperties

The tool removes the unevaluatedProperties value, introduced in later versions of JSON Schema, as this is not supported in OAS 3.0 JSON Schema Draft 7 used in OAS 3.0.

    myResponse:
      title: My Response
      description: Response from an API operation
      type: object
      unevaluatedProperties: false
      allOf:
        ...

becomes

    myResponse:
      title: My Response
      description: Response from an API operation
      type: object
      allOf:
        ...

The tool removes any $id or $schema keywords that may appear inside schema objects.

⤓ Convert $comment to x-comment

JSON Schema introduced $comment in schemas in 2020-12. Since OAS 3.0 uses JSON Schema Draft 4, and some tools will flag $comment as invalid, this tool removes these comments.

An earlier version of the tool converted $comment to x-comment However, other tools which do not allow $comment may not not support x-comment either.

Use the --convert-schema-comments CLI option or set convertSchemaComments to true in the Converter constructor options to requst conversion of $comment to x-comment rather than deleting $comment.

For example,

    Problems:
      title: Problems
      description: Problem Items
      type: array
      maxItems: 1000
      $comment: >-
        The value 1000 here must match `maxItems` in the ProblemList schema.
      items:
         $ref: '#/components/schemas/apiProblem'

becomes

    Problems:
      title: Problems
      description: Problem Items
      type: array
      maxItems: 1000
      x-comment: >-
        The value 1000 here must match `maxItems` in the ProblemList schema.
      items:
         $ref: '#/components/schemas/apiProblem'

Convert contentEncoding: base64 to format: byte

JSON Schema Draft 7 and later uses contentEncoding to specify [the encoding of non-JSON string content] (https://json-schema.org/understanding-json-schema/reference/non_json_data). Draft 4 supports format: byte for Base64 encoded strings.

This tool converts type: string schemas as follows:

2
OAS 3.1 schema OAS 3.0 schema 
type: string
contentEncoding: base64

type: string
format: byte

type: string
contentMediaType: 'application/octet-stream'

type: string
format: binary

Unsupported down conversions

Currently, the tool does not support the following situations. Contributions welcome!

  • openapi-down-convert does not convert exclusiveMinimum and exclusiveMaximum, unevaluatedProperties, patternProperties, propertyNames as defined in JSON Schema 2020-12; these are not supported in JSON Schema Draft 7 used in OAS 3.0
  • Webhooks are not removed. Contributions welcome!
  • The tool only supports self-contained documents. It does not follow or resolve external $ref documents embedded in the source document.
  • Request body and response body content object transformations, such as reversing content: { 'application/octet-stream': {} } as described in Migrating from OpenAPI 3.0 to 3.1.0
  • Converting other contentEncoding values (7bit, 8bit, binary, quoted-printable, base16, base32) (Note: contentEncoding: base64 is supported by converting to format: byte as listed above.)
  • Converting contentMediaType: 'type/subtype to media: { type: 'type/subtype'} for non-JSON data.